A magneto-optical memory apparatus of the type above-mentioned is arranged such that the optical head projects a laser light condensed to a spot having a diameter of about 1 .mu.m, onto a magneto-optical recording medium capable of recording and erasing information, causing the optical head to record information on the recording medium, as well as to read and erase the information recorded therein, and that the signal detector circuit produces a digital reproduction signal based on a reproduction signal supplied from the optical head. The signal detector circuit has a peak position detector circuit for detecting the peak positions of information pulses of the reproduction signal, or an amplitude detector circuit for detecting the width and interval of the information pulses. By either one of these circuits, a digital reproduction signal may be obtained from the reproduction signal.
As shown in FIG. 13, a conventional magneto-optical memory apparatus includes a magneto-optical disk 101 as the magneto-optical recording medium, and an optical head 102 for recording information in the magneto-optical disk 101, reading the recorded information and erasing the information.
The optical head 102 includes a semiconductor laser 111, a beam splitter (half-mirror) 112, an objective lens 113, a half-wave plate 116, an analyser 117, a condensing lens 114 and a photo detector 115. A light beam emitted from the semiconductor laser 111 passing through the beam splitter (half-mirror) 112 is condensed by the objective lens 113 and projected onto the magneto-optical disk 101. The reproduction light reflected from the magneto-optical disk 101 passes through the objective lens 113 and is vertically turned, at a portion thereof, by the beam splitter (half-mirror) 112. This light is inclined at a predetermined angle in the polarization direction by the half-wave plate 116. Then, the light passes through the analyser 117 and is condensed by the condensing lens 114. Thereafter, the light is incident on the photo detector 115. In the photo detector 115, the reproduction light is converted into an electric signal, which is then sent to a signal detector circuit 23. In the signal detection circuit 23, the data are reproduced.
The magneto-optical disk 101 as the magneto-optical recording medium, includes a magnetic thin film having a magnetization facilitating axis in a direction at a right angle to the surface of the thin film. Data may be recorded in the magneto-optical disk 101 as outlined below.
When the optical head 102 projects a laser light to the magnetic thin film of the magneto-optical disk 101, the portion of the thin film irradiated by the laser light is locally increased in temperature to lower the coercive force thereof. When a magnetic field is externally applied to this portion, the magnetization direction thereof may be inverted in a desired direction. Data are recorded by such inversion of the magnetization direction.
On the other hand, data reproduction may be carried out as outlined below.
When a laser light of linear polarization weaker than that used at the time of recording, is projected onto the magneto-optical recording medium, the polarization plane of the reproduction light which is a reflected light or a transmission light from the magneto-optical recording medium, is rotated by a desired angle according to the magnetization direction of the magneto-optical recording medium. The rotation of the polarization plane results from the Faraday effect for the transmission light and from the Kerr effect for the reflected light. For example, it is now supposed that the vector of the reflected light in a magnetization direction of the magneto-optical recording medium is expressed by R+, while the vector of the reflected light in the inverted magnetization direction with respect to the first-mentioned magnetization direction is expressed by R-. In this case, the rotation angles of the polarization planes corresponding to these reflected lights R+ and R- with respect to the incident polarization plane, are respectively expressed by +.theta..sub.K and -.theta..sub.K ', as shown in FIG. 14. Provision is made such that the reproduction signal as an electric signal is obtained by detecting the detection polarization plane components R.alpha.+ and R.alpha.- of the reflected lights R+ and R- on the detection polarization plane set at 45.degree. with respect to the incident polarization plane. At this time, R.alpha.- and R.alpha.+ are in low and high levels of a binary code, respectively.
The following description will discuss in more detail the operation of the magneto-optical memory apparatus having the principle above-mentioned, with reference to FIG. 15(a).
Corresponding to digital data (a) shown in FIG. 15(a), recording marks (b) are recorded in the magneto-optical disk 101 by inversion of magnetization. It is now supposed that the vectors of the reflected lights from the recording marks are expressed by R-, while the vectors of the reflected lights from other non-marking parts than the recording marks are expressed by R+. Then, a reproduction signal (c) is obtained from the optical head 102 which has received the reflected lights from the magneto-optical disk 101. In the reproduction signal (c), negative-direction pulses are generated by the reflected lights R- from the recording marks, and high-level pulses are generated by the reflected lights R+ from the non-marking parts.
When the signal detector circuit 23 has a peak position detector circuit (not shown), the signal detector circuit 23 to which the reproduction signal (c) above-mentioned has been entered, supplies a digital reproduction signal (d) of which rise portions are identical with the negative-direction peak positions of the reproduction signal (c). On the other hand, when the signal detector circuit 23 includes an amplitude detector circuit (not shown), the signal detector circuit 23 supplies a digital reproduction signal (e) which is in a high level for a predetermined width to the negative-direction peak positions of the reproduction signal (c). Based on the digital reproduction signal (d) or (e), digital data (f) may be obtained as reproduction information.
A conventional optical memory apparatus comprises: an optical information recording medium in which information is recorded by forming physical concavo-convex portions therein; an optical head for reading the information recorded in the optical information recording medium; and a signal detector circuit for generating a digital reproduction signal from the information read by the optical head. The description of the optical head and the signal detector circuit in the optical memory apparatus is here omitted since they have the same functions and the like as those of the optical head and the signal detector circuit in the magneto-optical memory apparatus above-mentioned.
The following description will discuss the operation of the optical memory apparatus with reference to FIG. 15(b).
As shown in FIG. 15(b), recorded in the optical information recording medium are recording marks (b), as recording information, corresponding to the digital data (a). In reading the recording marks by the optical head, the amounts of the reflected lights are increased or decreased by the diffraction thereof due to the concavo-convex portions above-mentioned. Thus, a reproduction signal (c) is generated. When the signal detector circuit has a peak position detector circuit, the signal detector circuit to which the reproduction signal (c) has been entered, generates a digital reproduction signal (d) of which rise portions are identical with the negative-direction peak positions of the reproduction signal (c). On the other hand, when the signal detector circuit includes an amplitude detector circuit, the signal detector circuit supplies a digital reproduction signal (e) which is in a high level for a predetermined width to the negative-direction peak positions of the reproduction signal (c). Based on the digital reproduction signal (d) or (e), digital data (f) may be obtained as reproduction information.
The magneto-optic al memory apparatus mentioned earlier generally employs a magneto-optical recording medium in which formed are (i) physical concavo-convex portions presenting track address information and the like and (ii) recording areas for recording information by inverting the magnetization direction of the magneto-optical recording medium.
The conventional above-mentioned arrangement presents the problem that it is difficult to enhance the recording density in the magneto-optical recording medium to increase the recording capacity.
More specifically, to enhance the recording density of the magneto-optical recording medium in the arrangement above-mentioned, the recording marks need to be further reduced in size. However, as the recording marks are reduced in size, it becomes difficult to record such recording marks into the magneto-optical recording medium. For example, when a laser light is condensed on the magneto-optical recording medium to record the recording marks, it is more difficult to properly form smaller recording marks in view of the degree of diaphragming the laser beam, the characteristics of the magneto-optical recording medium, the ambient temperature and the like. Accordingly, it is difficult to enhance the recording density by forming small-size recording marks. Thus, the conventional arrangement is not suitable to enhance the recording density.